Carrier frequency signal system



Jan. 7, 1941. W, HAHNLE 2,227,902

CARRIER FREQUENCY SIGNAL SYSTEM Filed May 28, L938 1149 AMPLIFIERDEMODULATUR V DM c S --0 L2 5 5p, 1/, 3 BP; Ph V2 CORRECTING BAND AMPL.AMPL; BAND PHASE AMPL/F/ER NETWORK PASS L/M/TE/Z P1455 SHIFTER(COMPENSATOR) FILTER TER I NV EN TOR.

ATTORNEY.

Patented Jan. 7, 1941 CARRIER FREQUENCY SIGNAL SYSTEM Walter Hithnle,Berlin-Siemensstadt, Germany,

assignor to Siemens & Halske, Aktiengesellschaft, Siemensstadt-Berlin,Germany Application May 28, 1938, Serial No. 210,584 In Germany June 18,1937 4 Claims.

In the transmission of broadcast programs and the like it is known inthe art totransmit one entire sideband and only a part of the othersideband resulting from the modulation of a car- 5 rier wave with asignal band. Such a method is particularly suited for the transmissionof intelligence of carrier frequency in which the lowest modulationfrequencies are around zero or but little different therefrom, as, forinstance, in

telegraphy, picture telegraphy and television.

This method of transmission offers practical advantages in both radioand wire transmission. For instance, the requisite frequency band is ofless width, while the risk of the sidebands being cut oif orextinguished as a result of differences in transit time is obviated.

In the demodulation of a message transmitted with partial suppression ofsideband, it is possible to obtain freedom from distortion,theoretically, by an ideal rectifier Without addition of the carrierwhen the modulation percentage is very low, and when no appreciabledifferences in transit time (phase) arise. However, it is uneconomicalto operate with low modulation percentages, because of the amplifierpower which is required. On the other hand, when the modulationpercentage is high, the carrier must be added upon demodulation.

If transmission of signals is concerned in which the relation betweenthe carrier frequency and the neighboring sideband corner frequency (i.e., its nearest sideband frequency), differs but slightly from unity, inother words, systems in which the lowest modulation frequency is equalto zero .or but little different from zero, the

carrier to be added, especially where the carrier frequency is high,cannot be. obtained by conventional methods. Moreover, the requirementsregarding the carrier frequency to be added are essentially more severein comparison with normal telephone signal transmissions. For instance,in television transmission, a phase distortion of not over :10 percentmay be regarded as admissible if the lowest modulation frequencycontained in the television band is equal to 25 cycles. The assumptionis here made that the frequenciesfrom 0 to 4 cycles pertaining to theso-called average picture brightness (background brightness) are sent'over a separate channel.

Since the carrier frequency itself may fluctuate around 10.5 percentabout its rated value be cause the stability of operation of oscillatorvalves as a general rule is not greater, the usual mode of producing thecarrier by means of filter- 5 ing in very narrow bandpass filters hasbecome impossible. Owing to the demands made as regards the phase,moreover, it does not appear possible to make available at the receivingend a. carrier presenting the correct phase relation; also, thetransmission of carriers of a different frequency fails to lead to thedesired success, among other reasons because of the demands respectingtransit time.

Now, in order to allow the practical use of carrier-frequencytransmission systems in which one sidebandis wholly andthe other onepartially transmitted, while at the same time excluding such drawbacksas inhere in known carrier generation methods, the carrier frequency tobe added, according to the invention, is obtained from the transmissionor signal band through distortion-corrector networks, bandpass filters,and amplitude limiting means with practically constant amplitude,without additional frequency modulation. If desired, the ensuingcarrierfrequency may be fed to a phase shifter of such action that, inthe presence of a departure of the carrier frequency from. its rated orassigned value, the ensuing phase difference is compensated. The meansrequired for the production of the carrier as a whole are soproportioned that there results no phase rotation for the carrierfrequency as long as it has exactly its assigned or rated value. Ifneeded, a further phase rotating (shifting) element must be added.

Inbrief, it may be said that the present invention concerns ademodulation system Where an exalted carrier is employed for producingdistortionless demodulation. The present invention The invention shallbe explained more fully by reference to the accompanying schematicillustrations wherein Fig. 1 illustrates applicants system, and Fig. 2illustrates one form of which the phase shifter Ph of Fig. 1 may take.In referring to Fig. 1, the signal band coming in, for instance, fromthe line L is fed through an amplifier V to the demodulator DM. By thecorrecting network or compensator E the aggregate attenuation of thefrequencies located in the vicinity of the carrier frequency F, say, inthe case of television work a band F1210 cycles per second, is to berendered stable in order that in this range of frequency there may beavailable only an amplitude modulated carrier without additionalfrequency modulation. By the bandpass BP1, a band symmetric to thecarrier F, in the case of television, a band of around 4x10 cycles persecond is filtered out, whence, optionally after amplification byamplifier V1, the amplitude limiter B and bandpass filter BPz, a carrierfrequency of practically constant amplitude is obtained. The bandpassfilter BP2 cuts out a. band that is still narrower than the band out outby the bandpass filter BP1, in fact, it is around one order of magnitudesmaller. It could be dispensed with, if no accompanying modulation arosein the carrier frequency through amplitude limitation. l'he elementsfrom the line end L as far as the bandpass filter BPz are so chosen thatthere is occasioned no phase rotation for the carrier frequency if it isexactly of the rated value. How-- ever, if the carrier frequency departsfrom the rated value, then the phase discrepancy is proportional to thefrequency drift. According to the design of the bandpass filters theremay thus result a departure of degrees for 310.5 percent frequencydeparture. This phase rotation is eliminated by the phase shifter Ph;the latter may be of the simple design shown in Fig. 2.

Referring to Fig. 2, it will be apparent that in the case of an undulyhigh carrier frequency, the potential at point D lags behind the voltageat point a; similarly, the voltage at a lags behind the voltage at theinput end of the distortion compensator. If the angles are small, then,by suitable .choice of the number of turns of the differentialtransformer DU, the no-load voltage at 0 may be set so as to be in phasewith the voltage at the input of the compensating network. Hence, thephase lagger, in the presence of no load or of negligible load, inside anarrow frequency range (for instance, rated frequency *0.5 percent)causes the transit time between the input to the compensating networkand the input to the demodulator to be equal to zero. The pairs ofelements I, 2; 3, 4; and 5, 6 are tuned to the carrier frequency so thatthe voltage at b is in phase with the voltage at a. .at the carrier.With departure from the carrier frequency, a phase shift is producedbetween a and b which is similar to that which is produced between theinput to E and the input to Ph, and by means of transformer DU thisphase shift is reversed. The voltage difference from a tob (due to phaseshift) being similar or equal to the voltage difference from line L to a(input of Ph), this voltage difference may be doubled by transformeraction and reversed and added on to the output of Ph, whereby there isneutralized or cancelled out the voltage produced by phase shift all theway from L to b.

The output of the phase shifter, optionally after amplification inamplifier V2, is fed to the demodulator DM in the form of a carrierfrequency; the demodulator output is fed to other receiver stages.

However, the compensator E will only roughly smooth the attenuationinside the channel of the bandpass filter BP1 if of standard design.Above this bandpass, even if the compensator is free from faults, afrequency modulation will exist or subsist side by side with amplitudemodulation if the carrier frequency departs from the rated value. To besure, the amplitude limiter 13 will almost completely eliminate theamplitude modulation, but the frequency modulation persists. Now, thenext bandpass filter BP2 is still narrower than the bandpass filter BP1and it .cuts down the frequency modulation by approximately 5 Nepers,occasioned by the band pass filter BP1 in case there is a shift incarrier frequency. Still, a residue will remain, in fact, another slightfrequency modulation is newly added since the carrier, as will be noted,is slightly dissymmetric in reference to the bandpass BPz. Hence, undercertain circumstances it may be expedient, especially when a push-pullchopper is not used for demodulation, to insert a further amplitudelimiter and another bandpass filter.

In order that the method here disclosed for carrier production may beparticularly advantageous, the attenuation curve for the aggregatesignal band should conveniently be so chosen that the condition where e+e :constant, where b1 and bn are the attenuations at two arbitrarypoints symmetric to the carrier frequency F2.

What is claimed is:

1. In a signal receiving system, a demodulator. means for impressing thereceived signal on the demodulator, and separate means for selectingfrom said signal a band of frequencies including the carrier, limitermeans for substantially removing amplitude modulation from the ratiofrequency energy in said band, a band pass filter for passing thecarrier but rejecting side band frequencies produced by said limiter,and means for impressing the energy passed by said band pass filter uponsaid demodulator with amplitude large compared to the amplitude of andin predetermined phase relation to the carrier impressed on thedemodulator by said first named means.

2. The method of obtaining a modulation free carrier from a broadspectrum of frequencies including a carrier, which includes the steps of1 3. In a demodulation system where an ex- L alted carrier is employedfor producing distortionless demodulation, the method of obtaining anexalted carrier which comprises selecting from a spectrum of frequenciesa narrow range of frequencies including the carrier, limiting theamplitude of energy in said range, and rejecting from said amplitudelimited energy side frequencies produced by the limiting action.

4. In a system for receiving a signal having one side band partiallysuppressed, an element upon which the received signal is impressed, ademodulator, a connection from said element to said demodulator, andanother connection from said element to said demodulator including inthe order to be named a network for restoring substantial equalitybetween the upper and lower side frequencies near the carrier, wherebythe carrier and the substantially equal upper and lower side frequenciesrepresent a purely amplitude modulated wave, a band pass filter, and anamplitude limiter. v

WALTER HAHNLE.

